Photoconductive elements useful, for example, in electrophotographic copiers and printers are composed of a conducting support having a photoconductive layer that is insulating in the dark but becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in non-irradiated areas a charge pattern known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (also called multi-active elements) have at least one charge-generation layer and at least one n-type or p-type charge-transport layer. Under actinic radiation, the charge-generation layer generates mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
Also useful in photoconductive elements are charge barrier layers, which are formed between the conductive layer and the charge generation layer to restrict undesired injection of charge carriers from the conductive layer. Various polymers are known for use in barrier layers of photoconductive elements. For example, Hung, U.S. Pat. No. 5,128,226, discloses a photoconductor element having an n-type charge transport layer and a barrier layer, the latter comprising a particular vinyl copolymer. Steklenski, et al. U.S. Pat. No. 4,082,551, refers to Trevoy U.S. Pat. No. 3,428,451, as disclosing a two-layer system that includes cellulose nitrate as an electrical barrier. Bugner et al. U.S. Pat. No. 5,681,677, discloses photoconductive elements having a barrier layer comprising certain polyester ionomers. Pavlisko et al, U.S. Pat. No. 4,971,873, discloses solvent-soluble polyimides as polymeric binders for photoconductor element layers, including charge transport layers and barrier layers.
The known barrier layer materials have certain drawbacks, especially when used with negatively charged elements having p-type charge transport layers. Thus, a negative surface charge on the photoconductive element requires the barrier material to provide a high-energy barrier to the injection of positive charges (also known as holes) and to transport electrons under an applied electric field. Many known barrier layer materials are not sufficiently resistant to the injection of positive charges from the conductive support of the photoconductive element. Also, for many known barrier materials the mechanism of charge transport is ionic. This property allows for a relatively thick barrier layer for previously known barrier materials, and provides acceptable electrical properties at moderate to high relative humidity (RH) levels. Ambient humidity affects the water content of the barrier material and, hence, its ionic charge transport mechanism. Thus, at low RH levels the ability to transport charge in such materials decreases and negatively impacts film electrical properties. A need exists for charge barrier materials that transport charge by electronic as well as ionic mechanisms so that films are not substantially affected by humidity changes.
Still further, a number of known barrier layer materials function satisfactorily only when coated in thin layers. As a consequence, irregularities in the coating surface, such as bumps or skips, can alter the electric field across the surface. This in turn can cause irregularities in the quality of images produced with the photoconductive element. One such image defect is caused by dielectric breakdowns due to film surface irregularities and/or non-uniform thickness. This defect is observed as toner density in non-image areas, also known as background density.
Photoconductive elements typically are multi-layered structures wherein each layer, when it is coated or otherwise formed on a substrate, needs to have structural integrity and desirably a capacity to resist attack when a subsequent layer is coated on top of it or otherwise formed thereon. Such layers are typically solvent coated using a solution with a desired coating material dissolved or dispersed therein. This method requires that each layer of the element, as such layer is formed, should be capable of resisting attack by the coating solvent employed in the next coating step.
Accordingly, a need exists for a negatively chargeable photoconductive element having a p-type photoconductor, and including an electrical barrier layer that can be coated from an aqueous or organic medium, that has good resistance to the injection of positive charges, can be sufficiently thick that minor surface irregularities do not substantially alter the field strength, and resists hole transport over a wide humidity range. Still further, a need exists for photoconductive elements wherein the barrier layer is substantially impervious to, or insoluble in, solvents used for coating other layers, e.g., charge generation layers, over the barrier layer.
Photoconductive elements comprising a photoconductive layer formed on a conductive support such as a film, belt or drum, with or without other layers such as a barrier layer, are also referred to herein, for brevity, as photoconductors.